The use of threaded tubular connections for joining flow conduits in an end-to-end relationship to form a continuous flow path for transporting fluid under pressure is well known. Oilfield tubular goods all use threaded connections for connecting adjacent sections of conduit or pipe. Examples of such threaded end connections designed for use on oilfield tubular goods are disclosed in U.S. Pat. Nos. 2,239,942; 2,992,019; 3,359,013; RE 30,647; and RE 34,467, all of which are assigned to the assignee of the present invention.
In U.S. Pat. No. RE 30,647 by Blose, a particular thread form is disclosed for a tubular connection that provides a strong joint while controlling the stress and strain in connected pin and box members within acceptable levels. The pin member is equipped with at least one generally dovetail-shaped external thread whose width increases in one direction along the pin, while the box member is equipped with at least one matching generally dovetail-shaped internal thread whose width increases in the other direction. In this manner, the mating set of helical threads provides a wedge-like engagement of opposing pin and box flanks that limits the extent of relative rotation between the pin and box members and defines a forcible make-up condition that completes the connection. In this thread structure, the flank shoulder angles, as well as the thread width, can be used to control the stress and strain preload conditions induced in the pin and box members for a given make-up torque. Thus, by tailoring the thread structure to a particular application or use, the tubular connection or joint is limited only by the properties of the materials selected.
During make-up of a threaded connection in accordance with the teachings of the Blose reissue patent, it has been observed that both liquid and paste-like thread lubricants can temporarily become trapped in the helical clearance or gap formed between the roots and crests of the respective pin and box member threads. The trapped lubricant (also known as thread dope) can, under certain circumstances, produce a torque reading between the relatively sliding pin and box threads indicative of the torque at the made up condition, thereby providing a false indication that the joint has been fully made up. Thereafter, the temporarily trapped thread lubricant can bleed off through the helical clearance between the roots and crests reducing the preload stress and strain to such an extent that the anticipated performance level or strength of the joint cannot be achieved.
U.S. Pat. No. RE 34,467 by Reeves discloses an improvement to the thread structure disclosed in the Blose reissue patent. Specifically, the potential for false torque readings in the joint resulting from trapped thread lubricant in the clearance between the roots and crests of the threads is addressed. Reliance on the torque readings developed by the forcible make-up of the connection is necessary to insure that the design stress and strain preload conditions actually exist in the connection. The Reeves reissue patent discloses a thread structure wherein the box and pin threads are tapered, in addition to having thread widths that increase in opposite directions, so that the roots, crests, and flanks of the threads are moved into engagement as the joint is made up. The threads are particularly designed so that the complementary roots and crests move into engagement before both of the opposing stab and load flanks move into engagement, whereby the volume of lubricant in the clearance between the roots and crests is substantially reduced. In this manner, most of the thread lubricant is displaced to the helical clearance between the opposing load flanks and forms a long, very thin ribbon that has little, if any, effect on the proper make-up of the connection or the ability of the thread surfaces to form seals as they are moved together.
Because of imperfections in the machined, seal forming thread surfaces, like those described in the Reeves reissue patent, thread lubricant can become isolated between sealing surfaces within the tubular connection. Once rotation between the pin and box members has advanced until the thread lubricant entirely fills the isolated volume between the pin and box members, additional rotation will produce an increase in the pressure of the lubricant. This increased pressure can result in higher tangential (hoop) and radial stresses in the connection, particularly in harsh cold weather environments, such as the North Sea, which cause the lubricant to become hardened and more viscous.